it is very stable; the issue is performance... I get ridiculously low transfer speeds, peaking at 150 KB/sec on NFS, Samba or SSH transfers (a quick test with netcat confirms it's the network link).
The same client and server, if using a wired fxp interface over powerlines, yield 4 MB/sec (but that connection drops out when other devices are activated - different story).

OpenBSD and the wireless AP are 5 meters away and have line of sight between them.
Latency between the two is good and consistent (2-4 ms, 0.0% packet loss)
ifconfig rum0:

The only thing I find odd is: 'DS1 mode 11g' - why would 'media autoselect' settle for the (I'm guessing) slowest setting?
'ifconfig rum0 media' lists a large number of options, from DS1 to OFDM54 (I can paste it here if you want)

I could try and force it to e.g. OFDM54 but I'm wary of losing connectivity as I'm remote to the machine, so I figured I'd ask for opinions or pointers before I try that.
Thank you for reading this post. Any ideas?

When it comes to dbm status values reported by ifconfig(8), I can never remember if high=good or high=bad. However, the hardware is reporting that it trains at 1Mbps most likely because it cannot communicate with the access point at higher speeds.

Force higher speeds via mediaopt only when you have another way to reach the platform should your change cause a communication break. That means, another network interface, another person, or you at the remote site.

High = good, I'm pretty sure about that (typical readings in that building are 70-80 dBm, in other rooms further away from the AP).
So I used an at job 5 minutes in the future to reset the interface if things went wrong, and another to restart the server if that failed, 5 minutes later.
I applied the change remotely to force OFDM54 mode, and it came back after a few seconds.
I cannot actually test right now, but the latency over 1000 is consistently lower than in DS1 mode. Fingers crossed, I'll check it later today and report back.

So I ran a bandwidth test (netcat) having forced OFDM54... throughput was roughly 4x higher, from 160KB/sec to 600 KB/sec. Good progress!
Then I thought about transmit power... this "server" uses a cardbus extension for USB 2.0, and the card has a power socket. I have nothing I could plug in there, but...

At least I've now got 6x the original throughput, but it's still only 20% of what it can theoretically do and 25% of the 100 MBps wired interface going over power lines.
I'll see if I can find something else, but any other ideas are welcome.

Very interesting... I'd forgotten that these things existed.
I found another USB wifi adapter that uses the uath driver (under a pile of toys...), so I'll benchmark this tonight.
If all else fails, wireless bridges can be quite cheap.

It took me quite a while to figure out the right position. I had to configure both devices in the living room at a distance of about 3 meter. Then I moved the bridge to its final position. While watching the signal strength I played with small variations in direction till I found the one with about 90 - 95% signal strength.

__________________
You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump

But never mind for now. 1.1 MB/sec is slow but sufficient, and the link is stable.
Eventually I'll move the OpenBSD box close to the router and go wired again.
Clearly J65nko has higher-end h/w than I do